REPTILIAN AGE. JURASSIC PERIOD. 93 



Although apparently associated with an Ammonite {A. Henryi), and a small 

 Ostrea, this shell seems to present, as far as we have been able to see, the characters 

 of a true Unio. We are the more inclined to regard it as belonging to that genus, 

 in consequence of the fact that we also find m the same matrix a small Planorhis, 

 and apparently a Valvata, and a Viviparus. None of our specimens show the hinge 

 very satisfactorily, though in one left valve (PI. Ill, Fig. 13, c.) it is seen to be rather 

 thick, a little arched, and provided with a long, posterior lateral tooth, extended 

 parallel to the cardinal margin, from which it is separated by a deep groove, for 

 the reception of a similar tooth in the other valve. The anterior tooth is com- 

 pressed, irregular, somewhat corrugated, and located nearly under the beaks ; while 

 the ligament is in all respects apparently similar, in form and position, to that of 

 our recent Uniones. 



Specimens having the surface well preserved sometimes show very small, radiat- 

 ing wrinkles on the posterior side of the umbones, near the hinge ; in most cases, 

 however, these are obsolete. A small, obscure, linear ridge is also generally seen 

 extending from the back part of each beak obliquely backward and downward, just 

 within the prominent umbonal slopes. The minute concentric vrainkles are very 

 regular on the beaks. 



The specimens of this species we have seen were not broken directly from the 

 rock in place, but from the composition, color, &c. of the matrix, and the circum- 

 stances under which they were found, we are led to think they occur in a Jurassic 

 bed seen at the locality where they were found, though we are not positively satis- 

 fied they may not be Cretaceous or Tertiary species. They diff'er, however, speci- 

 fically from any of the forms yet known in these rocks in the northwest. 



Locality and j)ositio7i. — Southwest base Black Hills. Jurassic. (Type 196.) 



Family CRASSATELLID^. (See page 34.) 

 Genus ASTARTE, Soweebt. 



Synon, — Astarte, J. Soweebt, Min. Concli. II, 1816, tab. 137. — Lajonkaike, Monogr. in Mem. Soc. de Hist. Nat. de Par. 

 I, 1823, 129. — Fleming, Brit. An. 1828, 409.— Rang. Man. 1829, 314.— Desh ayes, Encyo. Meth. II, 

 1830, 76 ; III, 553.— Scacchi, Osservaz. Zoolog. 1833, No. 2.— Beonn, Leth, 1837, 374. 

 Tridonta, Schum. Essai, 1817, 146.— Mollee, Isis, 1832, 135. 



Crassina, Lamk. Hist. Nat. V, 1818, 554. — Feeussac, tab. Syst. 1821, p. xlii Schweigg, Natgsch. 1820, 



710.— Blain\^. Malacol. 1825, 557. 

 Nicania, Leach, Jour. Phys. I, 1819, 88, 465.-^Blainv. Malacol. 1825, 558. 

 ? GoodalUa, Tdkton, Brit. Bivalves, 1822, 77.— Fleming, Brit. An. 1828, 409 and 429.— Foebbs, Malac. 1838, 



48. 

 Mactrina, Beown, Brit. Conch. 1827, tab. xvi ; and Conch. Text-Book, 1833 (VI ed. 159). 

 Etym. — Astarte, the Syrian Venus. 

 Examp. — Venus Danmoniensis, Mont. 



Shell oval, subtrigonal, or suborbicular, thick, usually compressed, 

 closed. Surface smooth, or marked with concentric strise or undulations ; 

 ligament external ; lunule generally well defined. Hinge with two strong 

 diverging primary teeth in one valve, and one or two in the other. Im- 

 pressions of the adductor and pedal muscles deep and well defined; 

 pallial line simple. 



A few species have been referred to this genus from Devonian and Carboniferous 



